Crude glyceride oils, particularly vegetable oils, are typically refined by a multi-stage process. The first stage of this process typically is degumming by treatment with water or with a chemical such as phosphoric acid, citric acid, or acetic anhydride. Gums (or "phospholipids") include such substances as lecithin and cephalin. About 90% of gums present in crude glyceride oils are capable of being hydrated and therefore are easily removed by a water wash. The remaining 10% can be converted to hydratable forms by the use of phosphoric acid as the degumming agent. Although gums may be separated from the oil at this point or carried into subsequent phases of refining, oil which has been subjected to this degumming step is said to be "degummed" herein. Various chemicals and operating conditions have been used to perform hydration of gums for subsequent separation.
After degumming (or instead of degumming), the oil may be refined by a chemical process including neutralization, bleaching, and deodorizing steps. Alternatively, a physical process may be used, including a pretreating and bleaching step and a steam refining and deodorizing step. Regardless of the particular refining process, it is desirable to reduce the levels of phospholipids, soaps (e.g., sodium oleate), and detrimental metals, all of which can adversely affect colors, odors, and flavors in the finished oil. Such detrimental metals include calcium, iron, and copper, whose ionic forms are thought to be chemically associated with phospholipids (and, possibly, heavy metal soaps) and to negatively affect the quality and stability of the final oil product. It is also desirable to reduce the level of chlorophyll which, if remaining in the oil, can tend to impart an unacceptably high level of green coloring to the oil as well as possibly causing instability of oil upon exposure to light.
Efforts have been made to remove phospholipids, detrimental metal ions, and chlorophyll from oil. For example, U.S. Pat. No. 4,629,588 discloses the use of untreated amorphous silica, and U.S. Pat. No. 4,734,226 discloses the use of an organic acid-treated amorphous silica, as adsorbents of phospholipids and certain metal ions. According to the '226 patent, organic acids, such as citric, acetic, ascorbic, or tartaric acids, are contacted with amorphous silica in a manner which causes at least a portion of the organic acid to be retained within the pores of the silica. According to another patent, namely U.S. Pat. No. 4,781,864, an acid-treated amorphous silica adsorbent is capable of removing both phospholipids and chlorophyll from glyceride oil. According to this patent, a fairly strong acid having a pK.sub.a of about 3.5 or lower is contacted with amorphous silica, and the resulting acid-treated amorphous silica has a pH of 3.0 or lower. The acidic conditions during which the acid-treated amorphous silica is prepared tends to result in the precipitation of metal oxides, especially iron oxide, within the pores of the silica and around the silica particles.
Soaps have been removed from oil in the past by a water wash step of up to 15% (by volume) of the oil being purified. A drawback of this method is that the wash effluent water must be regenerated if it is to be used again in a subsequent stage. Accordingly, it is desirable to utilize an adsorbent which minimizes or eliminates the need for a water wash step for the removal of soap.
It is also desirable to utilize an adsorbent which is capable of reducing the levels of phospholipids, soaps, detrimental metals, and chlorophyll in refining oil. In addition, it is desirable to minimize the amount of adsorbent required, because the adsorbent is eventually separated from the oil before the oil is used. When less adsorbent is used, filtration of the adsorbent is easier and less energy-intensive and tends to minimize oil losses in the filtercake.